Treatment of various conditions using balloon dilation catheters, for example, so-called Percutaneous Transluminal Angioplasty Catheters (i.e., “PTACs”), has progressed into narrower and more remote vessels within the body. This progression has necessitated the use of catheters having correspondingly smaller shaft diameters and longer shaft lengths. The trend towards catheters with smaller diameter, longer shafts has introduced additional concerns. The inflation/deflation time performance (i.e., the time required for inflation and deflation of the balloon) has increased with the use of longer, smaller diameter catheters. Deflation time for a dilation balloon can be significant, especially when it is desired to move the catheter to different locations to expand different portions of a body lumen, for example, a blood vessel. Additionally, the operating pressure of dilation balloons continues to rise. At one time a balloon inflation pressure of 10 atmospheres was considered high. Now, dilation balloons with operating pressures of up to 30 atmospheres are known, and it is foreseeable that even higher pressures may be utilized in the near future.
One conventional design of balloon catheter shafts is the coaxial design, wherein two concentrically disposed tubular members form the catheter shaft. In coaxial catheter shafts, the inside of the inner tubular member is used for the guidewire lumen, while the outer tubular member is used for the catheter shaft body. A jacket may be positioned over the outer tubular member to provide desired surface properties. The annular space between the outer surface of the inner tubular member and the inner surface of the outer tubular member forms an inflation/deflation lumen for transporting saline solution, contrast media or other non-compressible fluid for inflating and deflating the balloon.
The coaxial shaft design is considered to maximize the cross-sectional area available for the inflation/deflation lumen, thereby providing the best inflation/deflation performance for a given length catheter. The incompressible fluid used to inflate the balloon may be introduced into the balloon through the annular space between the outer surface of the inner tubular member and the inner surface of the outer tubular member or through radial ports or holes formed in the distal end of the outer tubular member and positioned inside the balloon.
However, fluid flow in the vicinity of the interconnection between a dilation balloon and a coaxial catheter shaft may be problematic, especially during the deflation phase of a procedure. To deflate a dilation balloon, the incompressible fluid used to inflate the balloon must be withdrawn, in some cases under negative pressure to decrease the amount of time required to deflate the balloon. Since a dilation balloon is typically formed from a thin, flexible polymer, the balloon collapses unpredictably during the deflation process. The collapsed balloon may partially or completely block the annular space between the inner tubular member and the outer tubular member and/or the radial inflation ports formed in the outer tubular member before the balloon is fully deflated. Further, conventional coaxial catheter shaft designs require that the balloon be bonded to the outer tubular member with a portion of the outer tubular member (i.e., the portion with the radial inflation ports) extending into the balloon. The portion of the outer tubular member extending into the balloon increases the diameter of the balloon in its deflated configuration, which in turn, requires the use of a larger introducer to insert the balloon into a body lumen.
Thus, there exists a need for a coaxial catheter shaft that provides for rapid deflation and for a reduced diameter of the balloon/catheter assembly in a deflated state.